Auto merge of rust-lang#32034 - alexcrichton:old-x86-msvc, r=aturon

rustc: Add an i586-pc-windows-msvc target

Similarly to rust-lang#31629 where an i586-unknown-linux-gnu target was added, there is
sometimes a desire to compile for x86 Windows as well where SSE2 is disabled.
This commit mirrors the i586-unknown-linux-gnu target and simply adds a variant
for Windows as well.

This is motivated by a recent [Gecko bug][ff] where crashes were seen on 32-bit
Windows due to users having CPUs that don't support SSE2 instructions. It was
requested that we could have non-SSE2 builds of the standard library available
so they could continue to use vanilla releases and nightlies.

[ff]: https://bugzilla.mozilla.org/show_bug.cgi?id=1253202

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